Do you want to (or even need to) access any WordPress capabilities from your bbpress install?

-In addition to wrapping bbPress with your WordPress template, do you actually need to use any WordPress built in functionality, or would it be easier to recreate the theme in bbPress and avoid the additional overhead of loading the entire wpmu system when your forums are accessed?

If you answered yes to the first question, then yes you should share the database. If you answered yes to the second question, explore the third before deciding on whether to integrate or not. If you decide to integrate, then the process is handled as you are installing, the part you would do after is ‘deep integration’ and you can’t do this before, it has to be done after the bb-config.php file is created.